Analogs of farnesylcysteine induce apoptosis in HL-60 cells

Severe hypocholesterolaemia is often neglected in haematological malignancies

AIM OF THE STUDY

It is generally believed that high levels of cholesterol (hypercholesterolaemia) are life-threatening, while low levels seem to be positive. Unfortunately this assumption is far from true, and can be indicative of an underlying serious medical condition in most of the cases (i.e. cancer). However, the biological role of severe hypocholesterolaemia is poorly understood. Here, the possible biological process is being investigated. Cholesterol plays a key role in cell proliferation, hence it has been suggested that low cholesterol levels are probably linked to the high cellular cholesterol demands from neoplastic cells.

SUMMARY OF THE METHODS

We used serum and isolated T-lymphocytes from patients with acute lymphoblast leukaemia and human lymphoblast cell line to test this hypothesis.

RESULTS

We found that patients with low serum cholesterol levels have instead high levels of cholesterol in lymphocytes. These data were supported with in vitro studies. In fact we have demonstrated that low cholesterol level in the culture medium was related to the neoplastic cellular growth, suggesting a greater use by lymphoma cells for their proliferation. Therefore by inhibiting cholesterol synthesis by mevastatin, in vitro, we showed that cholesterol levels did not change significantly in culture medium and the cellular growth was inhibited.

CONCLUDING STATEMENT

Following these preliminary results, blood cholesterol levels could be potentially considered a good biological marker to follow up the neoplastic process.

Role of transmethylation reactions in alcoholic liver disease

Alcoholic liver disease is a major health care problem worldwide. Findings from many laboratories, including ours, have demonstrated that ethanol feeding impairs several of the many steps involved in methionine metabolism. Ethanol consumption predominantly results in a decrease in the hepatocyte level of S-adenosylmethionine and the increases in two toxic metabolites, homocysteine and S-adenosylhomocysteine. These changes, in turn, result in serious functional consequences which include decreases in essential methylation reactions via inhibition of various methyltransferases. Of particular interest to our laboratory is the inhibition of three important enzymes, phosphatidylethanolamine methyltransferase, isoprenylcysteine carboxyl methyltransferase and protein L-isoaspartate methyltransferase. Decreased activity of these enzymes results in increased fat deposition, increased apoptosis and increased accumulation of damaged proteins-all of which are hallmark features of alcoholic liver injury. Of all the therapeutic modalities available, betaine has been shown to be the safest, least expensive and most effective in attenuating ethanol-induced liver injury. Betaine, by virtue of aiding in the remethylation of homocysteine, removes both toxic metabolites (homocysteine and S-adenosylhomocysteine), restores S-adenosylmethionine level, and reverses steatosis, apoptosis and damaged proteins accumulation. In conclusion, betaine appears to be a promising therapeutic agent in relieving the methylation and other defects associated with alcoholic abuse.

Amide-substituted farnesylcysteine analogs as inhibitors of human isoprenylcysteine carboxyl methyltransferase

N-Acetyl-S-farnesyl-L-cysteine (AFC) is the minimal substrate for the enzyme isoprenylcysteine carboxyl methyltransferase (Icmt). A series of amide-modified farnesylcysteine analogs were synthesized and screened against human Icmt. From a 23-membered library of compounds, six inhibitors were identified and evaluated further. The adamantyl derivative 7c was the most potent inhibitor with an IC(50) of 12.4 microM.

Liver prenylated methylated protein methyl esterase is an organophosphate-sensitive enzyme

Prenylation and subsequent methylation are essential modifications on a significant proportion of eucaryotic proteins. Proteins such as the G-gamma subunits of G-protein coupled receptors, nuclear lamins, and guanine nucleotide-binding proteins such as Ras are prenylated and undergo methylation. Prenylated methylated protein methyl esterase (PMPMEase) readily hydrolyses the prenylated protein methyl esters, thus making this step reversible and possibly regulatory. Benzoyl-glycyl-farnesyl-cysteine methyl ester (BzGFCM) was developed as a specific PMPMEase substrate and characterized by electron spray ionization mass spectrometry (ESI-MS) to be of the calculated molecular mass. Rat liver and brain PMPMEase hydrolyzed BzGFCM, forming benzoyl-glycyl-farnesyl-cysteine (BzGFC) in a time- and concentration-dependent manner. Both enzymes cleaved BzGFCM with K(m) values of 4.58 +/- 0.30 and 25.57 +/- 2.36 microM and V(max) values of 2.21 +/- 0.03 and 0.17 +/- 0.003 nmol/min/mg, respectively. The liver enzyme eluted from a gel-filtration column as a single peak of apparent size, 89 kDa. The brain enzyme eluted as two main peaks of 53 and 890 kDa. Organophosphorus pesticides (OPs), which are suspected to be involved in human disorders such as parkinsonism, neuronal, and retinal degeneration, inhibited the liver enzyme with IC(50) values from 4.77 muM for parathion to 0.04 microM for paraoxon, respectively. Only about 25% of the brain enzyme was inhibited by 0.5-1 mM solutions of mipafox, while 0.1 and 1 mM paraoxon inhibited over 50% and 95% of the enzyme, respectively. Paraoxon is thus about 2,250 times less potent against the brain than the liver PMPMEase. BzGFCM was not hydrolyzed by various cholinesterases, indicating its specificity for PMPMEase. Perturbations in prenylated protein metabolism might play a role in noncholinergic OPs-induced toxicity, since prenylated proteins play such important roles in cell signaling, proliferation, differentiation, and apoptosis.

Inhibition mechanism of S-adenosylmethionine-induced movement deficits by prenylcysteine analogs

We previously showed that S-adenosylmethionine (SAM) induces movement impairments similar to those observed in Parkinson's disease (PD) apparently by prenylated protein methylation; 5 kDa molecules being methylated and the symptoms being inhibited by prenylcysteine (PC) analogs. In the present study, we explore the biochemical mechanism of action of the PC analogs. N-acetylgeranylcysteine (AGC), N-acetylfarnesylcysteine (AFC), N-acetylgeranylgeranylcysteine (AGGC), farnesylthioacetic acid (FTA), farnesyl-2-ethanesulfonic acid (FTE) and farnesylsuccinic acid (FMS), but not farnesylthiotriazole (FTT) and farnesylthiolactic acid (FTL), inhibited the SAM-induced motor impairments. Incubation of the respective analogs with rat brain membranes containing prenylated protein methyltransferase (PPMTase) resulted in the methylation of AGC, AFC and AGGC. FTA, FTE, FMS and FTT, but not FTL, inhibited the enzyme activity. A single injection of the active analogs remained effective for at least 3 days against repeated injections of 1 micromol SAM. Amphetamine-induced hyperactivity in rats was inhibited by SAM but potentiated by FTE. During 60 min, the movement time for amphetamine-treated rats was 1477 s compared with 633 and 1664 s for amphetamine+SAM- and amphetamine+FTE-treated rats, respectively. The total distance for amphetamine+FTE-treated rats was 82% higher than for amphetamine. The horizontal activity was 30,728 (amphetamine), 15,430 (FTE), 18,526 (amphetamine+SAM), 41,736 (amphetamine+FTE) and 7004 (SAM) as compared to the PBS control (4726). The intricate relationship between the actions of SAM, which speeds up prenylated protein methylation and impairs movement, amphetamine, which increases synaptic dopamine levels and movement, and the PC analogs, which prevent the SAM-induced movement impairments, suggests a SAM-induced defect on dopamine signaling as the likely cause of the symptoms. The data reveal that interaction of PC analogs with PPMTase may not be an indicator of anti-PD-like activity.

Studies of the isoprenoid-mediated inhibition of mevalonate synthesis applied to cancer chemotherapy and chemoprevention

Pools of farnesyl diphosphate and other phosphorylated products of the mevalonate pathway are essential to the post-translational processing and physiological function of small G proteins, nuclear lamins, and growth factor receptors. Inhibitors of enzyme activities providing those pools, namely, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase and mevalonic acid-pyrophosphate decarboxylase, and of activities requiring substrates from the pools, the prenyl protein transferases, have potential for development as novel chemotherapeutic agents. Their potentials as suggested by the clinical responses recorded in Phase I and II investigations of inhibitors of HMG CoA reductase (the statins), of mevalonic acid-pyrophosphate decarboxylase (sodium phenylacetate and sodium phenylbutyrate), and of farnesyl protein transferase (R115777, SCH66336, BMS-214662, Tipifarnib, L-778,123, and, prematurely, perillyl alcohol) are dimmed by dose-limiting toxicities. These nondiscriminant growth-suppressive agents induce G1 arrest and initiate apoptosis and differentiation, effects attributed to modulation of cell signaling pathways either by modulating gene expression, suppressing the post-translational processing of signaling proteins and growth factor receptors, or altering diacylglycerol signaling. Diverse isoprenoids and the HMG CoA reductase inhibitor, lovastatin, modulate cell growth, induce cell cycle arrest, initiate apoptosis, and suppress cellular signaling activities. Perillyl alcohol, the isoprenoid of greatest clinical interest, initially was considered to inhibit farnesyl protein transferase; follow-up studies revealed that perillyl alcohol suppresses the synthesis of small G proteins and HMG CoA reductase. In sterologenic tissues, sterol feedback control, mediated by sterol regulatory element binding proteins (SREBPs) 1a and 2, exerts the primary regulation on HMG CoA reductase activity at the transcriptional level. Secondary regulation, a nonsterol isoprenoid-mediated fine-tuning of reductase activity, occurs at the levels of reductase translation and degradation. HMG CoA reductase activity in tumors is elevated and resistant to sterol feedback regulation, possibly as a consequence of aberrant SREBP activities. Nonetheless, tumor reductase remains sensitive to isoprenoid-mediated post-transcriptional downregulation. Farnesol, an acyclic sesquiterpene, and farnesyl homologs, gamma-tocotrienol and various farnesyl derivatives, inhibit reductase synthesis and accelerate reductase degradation. Cyclic monoterpenes, d-limonene, menthol and perillyl alcohol and beta-ionone, a carotenoid fragment, lower reductase mass; perillyl alcohol and d-limonene lower reductase mass by modulating translational efficiency. The elevated reductase expression and greater demand for nonsterol products to maintain growth amplify the susceptibility of tumor reductase to isoprenoids, therein rendering tumor cells more responsive than normal cells to isoprenoid-mediated growth suppression. Blends of lovastatin, a potent nondiscriminant inhibitor of HMG CoA reductase, and gamma-tocotrienol, a potent isoprenoid shown to post-transcription-ally attenuate reductase activity with specificity for tumors, synergistically affect the growth of human DU145 and LNCaP prostate carcinoma cells and pending extensive preclinical evaluation, potentially offer a novel chemotherapeutic strategy free of the dose-limiting toxicity associated with high-dose lovastatin and other nondiscriminant mevalonate pathway inhibitors.

Isoprenylcysteine carboxyl methyltransferase activity modulates endothelial cell apoptosis

Extracellular ATP, adenosine (Ado), and adenosine plus homocysteine (Ado/HC) cause apoptosis of cultured pulmonary artery endothelial cells through the enhanced formation of intracellular S-adenosylhomocysteine and disruption of focal adhesion complexes. Because an increased intracellular ratio of S-adenosylhomocysteine/S-adenosylmethionine favors inhibition of methylation, we hypothesized that Ado/HC might act by inhibition of isoprenylcysteine-O-carboxyl methyltransferase (ICMT). We found that N-acetyl-S-geranylgeranyl-L-cysteine (AGGC) and N-acetyl-S-farnesyl-L-cysteine (AFC), which inhibit ICMT by competing with endogenous substrates for methylation, caused apoptosis. Transient overexpression of ICMT inhibited apoptosis caused by Ado/HC, UV light exposure, or tumor necrosis factor-alpha. Because the small GTPase, Ras, is a substrate for ICMT and may modulate apoptosis, we also hypothesized that inhibition of ICMT with Ado/HC or AGGC might cause endothelial apoptosis by altering Ras activation. We found that ICMT inhibition decreased Ras methylation and activity and the activation of the downstream signaling molecules Akt, ERK-1, and ERK-2. Furthermore, overexpression of wild-type or dominant active H-Ras blocked Ado/HC-induced apoptosis. These findings suggest that inhibition of ICMT causes endothelial cell apoptosis by attenuation of Ras GTPase methylation and activation and its downstream antiapoptotic signaling pathway.

Stereospecificity and kinetic mechanism of human prenylcysteine lyase, an unusual thioether oxidase

Prenylated proteins contain either a 15-carbon farnesyl or a 20-carbon geranylgeranyl isoprenoid covalently attached to cysteine residues at or near their C terminus. The cellular abundance of prenylated proteins, as well as the stability of the thioether bond, poses a metabolic challenge to cells. A lysosomal enzyme termed prenylcysteine lyase has been identified that degrades a variety of prenylcysteines. Prenylcysteine lyase is a FAD-dependent thioether oxidase that produces free cysteine, an isoprenoid aldehyde, and hydrogen peroxide as products of the reaction. Here we report initial studies of the kinetic mechanism and stereospecificity of this unusual enzyme. We utilized product and dead end inhibitors of prenylcysteine lyase to probe the kinetic mechanism of the multistep reaction. The results with these inhibitors, together with those of other experiments, suggest that the reaction catalyzed by prenylcysteine lyase proceeds through a sequential mechanism. The reaction catalyzed by the enzyme is stereospecific, in that the pro-S hydride of the farnesylcysteine is transferred to FAD to initiate the reaction. With (2R,1'S)-[1'-(2)H(1)]farnesylcysteine as a substrate, a primary deuterium isotope effect of 2 was observed on the steady state rate. However, the absence of an isotope effect on an observed pre-steady-state burst of hydrogen peroxide formation implicates a partially rate-determining proton transfer after a relatively fast C-H (C-D) bond cleavage step. Furthermore, no pre-steady-state burst of cysteine was observed. The finding that the rate of cysteine formation was within 2-fold of the steady-state k(cat) value indicates that cysteine production is one of the primary rate-limiting steps in the reaction. These results provide substantial new information on the catalytic mechanism of prenylcysteine lyase.

Farnesylpyridinium, an analog of isoprenoid farnesol, induces apoptosis but suppresses apoptotic body formation in human promyelocytic leukemia cells

1-Farnesylpyridinium (FPy), an analog of isoprenoid farnesol, initially induced morphological changes similar to those of typical apoptosis in human leukemia HL-60 cells but FPy-treated cells were characterized by the absolute absence of final apoptotic events such as fragmentation into apoptotic bodies. FPy-induced cell death was considered to be apoptotic on the basis of the induction of DNA fragmentation and the protection against these events by the coaddition of a pan-caspase inhibitor. The increase in the cytoplasmic cytochrome c level supported the possibility that FPy-treated cells should have the ability to complete the entire apoptotic process ending in cell fragmentation and apoptotic body formation. At concentrations too low to induce apoptosis, FPy could suppress the induction of apoptotic body formation in HL-60 cells by typical inducers of apoptosis such as actinomycin D or anisomycin. FPy exhibited a cytochalasin-like effect on spatial arrangement of actin filament independent of its apoptosis-inducing activity.

Involvement of Rho GTPases in the transcriptional inhibition of preproendothelin-1 gene expression by simvastatin in vascular endothelial cells

Endothelial dysfunction is characterized by an impaired vasodilatory response to endothelial agonists as well as by alterations in adhesion and coagulation processes. 3-Hydroxy-3-methylglutaryl-CoA reductase inhibitors (statins) have been shown to be useful in the reversal of endothelial dysfunction, an effect that may be independent of the reduction in cholesterol levels. Both the L-arginine-nitric oxide-cGMP and endothelin pathways are involved in the regulation of vascular tone. Here, we show that the basal transcription rate of the preproendothelin-1 gene was decreased by simvastatin (10 micromol/L) in bovine aortic endothelial cells. Transfection studies with the preproendothelin-1 gene promoter showed that mevalonate (100 micromol/L) was able to prevent the inhibitory effect mediated by simvastatin. Protein geranylgeranylation, but not farnesylation, proved to be crucial for a correct expression of the preproendothelin-1 gene. The C3 exotoxin from Clostridium botulinum that selectively inactivates Rho GTPases, the processing of which involves geranylgeranylation, reproduced the inhibitory effect of simvastatin on the expression of preproendothelin-1. Overexpression of dominant-negative mutants of RhoA and RhoB led to a significant reduction in the preproendothelin-1 promoter activity, whereas the expression of wild-type and constitutively active forms of these proteins resulted in an increase, in support that Rho proteins are required for the basal expression of the preproendothelin-1 gene. Finally, we show that the Rho-dependent activation of the preproendothelin-1 gene transcription was inhibited by simvastatin. Thus, the control of vascular tone and proliferative response mediated by endothelin-1 is regulated at multiple levels, among which the Rho proteins play an essential role.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

Recent advances in the study of prenylated proteins

Post-translational modification of proteins with isoprenoids was first recognized as a general phenomenon in 1984. In recent years, our understanding, including mechanistic studies, of the enzymatic reactions associated with these modifications and their physiological functions has increased dramatically. Of particular functional interest is the role of prenylation in facilitating protein-protein interactions and membrane-associated protein trafficking. The loss of proper localization of Ras proteins when their farnesylation is inhibited has also permitted a new target for anti-malignancy pharmaceuticals. Recent advances in the enzymology and function of protein prenylation are reviewed in this article.

Synthesis and evaluation of homofarnesoyl-substituted CAAX-peptidomimetics as farnesyltransferase inhibitors and antiproliferative agents

Several CAAX-peptidomimetics were linked to homofarnesoic acid via a beta-alanyl spacer with the intention to obtain a novel type of bisubstrate analogue farnesyltransferase inhibitors. However, the compounds were found to be only weakly active in the farnesyltransferase inhibition assay. Nevertheless, they displayed antiproliferative activity against different tumor cell lines in the low micromolar range. Replacement of the beta-alanine moiety by aspartic acid-1-methyl ester resulted in a compound which inhibited the farnesyltransferase with an IC50 of 860 nM. The corresponding free acid showed a eightfold loss in activity (IC50 = 6.9 microM).

Induction of apoptosis in p53-null HL-60 cells by inhibition of lanosterol 14-alpha demethylase

To determine the role of cholesterol deprivation in cell proliferation and, eventually, in apoptosis, HL-60 promyelocytic cells were incubated in a cholesterol-depleted medium in the presence of SKF 104976, a specific inhibitor of lanosterol 14-alpha demethylase. As expected, SKF 104976 efficiently blocked the [14C]-acetate incorporation into cholesterol, whereas it induced the accumulation of both lanosterol and, especially, dihydrolanosterol. As a consequence, cell proliferation was greatly depressed at 24 h of treatment with the drug, and clear signs of apoptosis--annexin V binding, condensed and fragmented nuclei and DNA ladder--were observed thereafter. Provided that the HL-60 cell line does not express p53, it may be concluded that apoptosis induced by cholesterol deprivation is not dependent on this tumor suppressor protein. Supplementing the incubation medium with LDL-cholesterol or pure free cholesterol, fully prevented cell growth inhibition and apoptosis induction, whereas mevalonate was ineffective. These results indicate that cholesterol plays a specific role in cell proliferation, a function that is not shared by its precursors lanosterol and dihydrolanosterol.